I. Field of the Invention
The present invention relates to communication systems utilizing spread spectrum signals, and, more particularly, to a novel and improved method and apparatus for adaptive sectorization within a spread spectrum communication system.
II. Description of the Related Art
Communication systems have been developed to allow transmission of information signals from a base station location to a physically distinct user or subscriber location. Both analog and digital methods have been used to transmit such information signals over communication channels linking the base station and user locations. Digital methods tend to afford several advantages relative to analog techniques, including, for example, improved immunity to channel noise and interference, increased capacity, and improved security of communication through the use of encryption.
In transmitting an information signal in either direction over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave on the basis of the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the channel bandwidth. At the recipient location the original message signal is replicated from a version of the modulated carrier received subsequent to propagation over the channel. Such replication is generally achieved by using an inverse of the modulation process employed during message transmission.
Modulation also facilitates multiplexing, i.e., the simultaneous transmission of several signals over a common channel. Multiplexed communication systems will generally include a plurality of remote subscriber units requiring intermittent service rather than continuous access to the communication channel. Systems designed to enable communication with a selected subset of a full set of subscriber units have been termed multiple access communication systems.
A particular type of multiple access communication system, known as a code division multiple access (CDMA) modulation system, may be realized in accordance with spread spectrum techniques. In spread spectrum systems, the modulation technique utilized results in a spreading of the transmitted signal over a wide frequency band within the communication channel. Other multiple access communication system techniques, include time division multiple access (TDMA), and frequency division multiple access (FDMA). However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention.
In the above-referenced U.S. Pat. No. 4,901,307, a multiple access technique is disclosed where a large number of mobile system users each having a transceiver communicate through satellite repeaters or terrestrial base stations using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
For a particular cellular CDMA system, communication between a base station and subscriber units within the surrounding cell region is achieved by spreading each transmitted signal over the available channel bandwidth by using a unique user spreading code. In such CDMA systems the code sequences used for spreading the spectrum are constructed from two different types of sequences, each with different properties to provide different functions. For example, a first type of sequence used are the I and Q channel PN codes, which are shared by all signals in a cell or sector. In addition, each user may be identified by a unique long PN code, which is typically of a longer period than the I and Q channel PN codes.
Referring to FIG. 1, there is shown an exemplary CDMA communication cell 10 in which are disposed a plurality of fixed and mobile user subscriber units 12 and a base station 14. The subscriber units 12 are grouped into first, second and third user sectors 16, 18 and 20, each of which support an equivalent number of traffic channels. The base station 14 may include a set of fixed-beam antennas (not shown) dedicated to facilitating communication with subscriber units in each user sector. Alternatively, a three element antenna array could be utilized to divide the cell into the specified user sectors.
One advantage of the system of FIG. 1 is that the base station 14 typically includes a diversity receiver disposed to separately receive individual multipath echoes of the PN spread waveform transmitted by each subscriber unit 12. Multipath echoes may be created by reflection of the transmitted user signal from objects within the propagation environment. The individual multipath signals are then time aligned within separate receiver "fingers" dedicated to specific multipath signals, and are then combined in order to improve signal to noise ratio. When the cell 10 is partitioned into more than several sectors (e.g., six sectors), a relatively narrow beam is assigned to each sector. This increased sectorization may unfortunately hinder reception of those multipath signals outside each concentrated sector beam, thereby undesirably reducing signal to noise ratio.
Accordingly, it is an object of the present invention to provide an adaptive sectorization technique enabling separate tracking and reception of the direct and multipath signals transmitted to and from users within a digital communication system.